Control of magnetic excitation flux is not available in standard PM brushless dynamoelectric machines. Such control has only been possible with current vector control techniques or with the addition of a field excitation control winding that receives a control current to weaken or strengthen the primary PM flux. In the case of vector control, a motor controller operates as an inverter system to inject the direct axis (d-axis) current that weakens the PM flux to a desired degree. However, such d-axis current injection to control magnetic flux excitation has certain drawbacks, such as a significant increase in stator winding losses, resulting in excess heat dissipated in the stator winding and irreversible demagnetization of low energy density rotor PMs, such as rotor PMs of the ferrite type.
Machines that have an additional field excitation control winding do not suffer from such adverse effects. Thus, so-called hybrid electric machines with PMs and an additional field excitation winding for direct flux control may be a better choice for industrial motor drives and generators. However, most such hybrid electric machines have relatively complex designs with resulting increases in size, weight and expense.